A variety of paper and paperboard materials are used to manufacture cases, cartons, and trays for packaging purposes. Such paper and paperboard materials include virgin and recycled kraft, high and low density kraft, corrugated, and cartonstock also known as boxboard or chipboard, and various treated and coated variations of such.
Corrugated board is made of two paper components, namely linerboard and medium, held together by starch. The specific combination of medium and linerboards determines the type, strength and overall performance of the corrugated board. There are many different corrugated machines in production today. However, all corrugated board is manufactured using primarily the same production process.
In the basic process, a first roll of linerboard, typically the liner which will form the inside of the corrugated container, and a first roll of medium are mounted on roll stands. The medium is conditioned in a steam bath and then enters the corrugating rolls where it is pressed between two fluting rolls to form the intended flute pattern. The flute tips on one face of the medium are then coated with starch (aqueous glue) and brought in contact with the first liner. Starch is then applied to the medium flute tips on the opposing face which is then contacted to a second liner, typically the outside liner board. The composite of fluted medium sandwiched between two linerboards is then advanced to heated steam chests or hot plates where the starch is dried resulting in the fluted medium being adhesively bonded to the linerboards at the flute tips. For doublewall and triplewall board, each wall or layer is produced separately by first bonding the fluted medium to a single liner and then bonding the single layers to each other.
It is often desirable to increase the strength of the corrugated board. This is typically done with reinforcement strings consisting of a continuous fibrous substrate such as a yarn coated with a hot melt adhesive composition. The strings are positioned at various intervals on top of the flute tips, typically perpendicular to the fluting, prior to bonding the flute tips to the second linerboard.
Hot melt coated tapes are employed for reinforcement as well as for opening systems for corrugated and cartonstock containers, such as those typically found on laundry detergent boxes and various next day mail envelopes. For this application, the tape is positioned and bonded to the corrugated in such a manner to cleanly tear through the corrugated or boxboard layers. Tear tapes are generally multi-layered composites comprising a fibrous web coated with a hot melt adhesive which is bonded to either a paper or polyamide backing.
Tear opening tapes systems, such as those described in U.S. Pat. No. 5,098,757 issued Mar. 24, 1992, are typically incorporated in the corrugation process during what has commonly been called the “doublebacker” segment of the corrugated board forming machine. The doublebacker is the portion of the process wherein the preformed wall boards (liner/medium/liner) are bonded to each other.
Reinforcement strings and opening tapes are sold by H.B. Fuller Company under the trade names Sesame® Tape and String King® Reinforcement strings. All such products currently available employ hydrophobic hot melt adhesive compositions which are essentially unaffected by moisture or water. The heat of the corrugation process, which is used to dry the starch, melts or softens the hot melt adhesive composition on the coated tape or string to bond the fibrous core to the corrugated or cartonstock. The temperature and duration of time the corrugated materials are exposed to heat during the corrugation process varies widely. Therefore, a variety of tape and string products have been developed in order to match the melting temperature, open time, and viscosity of the hot melt adhesive on the tape or string with the conditions of the corrugation operation.